This invention relates to enclosures for providing a moisture free and particle free environment by coating an inner surface of a container with a composition having both moisture getter and particle getter properties. It is primarily concerned with maintaining a moisture free and particle free environment in casings and enclosures for semiconductor devices and integrated circuits.
It is difficult to manufacture a hermetic packaging for microelectronic devices with low water vapor content and to maintain the wafer vapor content at a low level during the device's useful lifetime. There are various channels by which water vapor finds its way to the inside of the enclosure:
1. The various seals in packagings are usually not perfect and ambient air, containing moisture, leaks inside the enclosure. PA0 2. Many epoxies used to bond dies and substrates outgas moisture in the packaging with time. PA0 3. The packaging material itself outgasses a certain amount of moisture. Baking prior to sealing is necessary to liberate all the absorbed moisture. PA0 4. The sealing atmosphere has to be moisture free. PA0 5. The testing process for leakage rate may itself introduce moisture inside the package if not done with extreme care.
Another problem that often arises in the manufacture of hermetic devices is the presence of unwanted solid particles on surfaces. Such particles may be constituted of metallic residues from solders or wirebonding, organic residue, from adhesives and fluxes, fibers from cleaning tools such as cotton swabs, airborne particles depositing on the surface, etc. These particles can mechanically damage the microcircuits in the device or cause electrical shorts (wires, solder, conductive epoxy). To minimize such potential problem, the manufacturing is carried out in a so-called "clean room", where the ambient air contains only a very small amount of solid particles. In addition, most processing steps have to be followed up by solvent cleaning to remove residual solid particles from the surfaces. Despite such extensive precautions, failure of parts due to particle contamination is still not entirely eliminated.
The prior art teaches various desiccating solids and liquids and some may be useful in electronic packaging applications. Thus, Aoki et al., U.S. Pat. No. 4,749,392, teach a moisture absorbent sealed between two plastic films of varying permeability. Anderson et al., U.S. Pat. No. 4,977,009, teach a moisture absorbing composite coating made with two solid layers. Plachendov et al., U.S. Pat. No. 3,704,806, disclose a dehumidifying mixture composed of zeolite and a thermosetting plastic binder, which was an improvement over the zeolite+silicone fluid mixture previously used. Evani et al. teach in U.S. Pat. No. 4,535,098 water absorbent compositions of colloidal silica in polymeric binders. Tokuyama et al. teach in U.S. Pat. No. 4,615,823 desiccating salts in a polymer with a fibrous additive. Booe teaches in U.S. Pat. No. 4,081,397 a desiccant for electronic devices which consists of alkaline earth oxides in an elastomeric matrix, such a silicone or polyurethane. Deffeyes teaches in U.S. Pat. No. 4,036,360 a package having a dessicant consisting of a dessicant material dispersed in a film-forming resin. The resin material is taught as a urethane, and the dessicant is taught as a chloride, sulfate, zeolite or clay. Taylor teaches in U.S. Pat. No. 4,013,566 a flexible solid desiccant comprising finely divided molecular seive particles in epoxy.
Some of these teachings may be useful to solve the moisture problem in hermetically sealed enclosures and electronic packages but none of them addresses the problem encountered with residual solid particles.